Yamamoto Haruyuki was originally from Mikawa Province. Like many notable warriors he became a buddhist monk and chose the name Kansuke. He started his career as a minor retainer of the Imagawa, who apparently didn't recognize his talents. He was partially lame and blind on one eye so probably didn't strike an imposing figure. Itagaki Nobutaka (killed Uedahara 1548) introduced him to Takeda Shingen, who was immediately impressed with him and gave him a 1,000-koku fief on the spot (this was later increased to 4000). He eventually became Takeda Shingen's gun-bugyo (army commisioner) and right-hand man, especially with regards to strategy.
Kansuke served the Takeda valuably in their conquest of Shinano province. After its capture in 1545, he was tasked with building up Takatő castle as the local Takeda headquarters and base for future campaigns into the Ina and Shiojiri districts. He designed a siege tower in use during one of the campaign's frequent sieges. He bribed or persuaded several families around Toishi Castle to go over to the Takeda side in 1550; and when counterattacked by the Murakami after the abortive siege of Toishi that year he is attributed with the tactic that saved the Takeda force (Sanada Yukitaka took the castle for the Takeda the next year). The work "Heiho Okugi Sho - The Secret of High Strategy" is attributed to him. This appears as a part of the Takeda family history Koyo Gunkan, and is, unlike the Gunkan, available in western languages.
The Takeda strategy at the famous "fourth battle of Kawanakajima", against Uesugi Kenshin, was designed by him. The battle is recorded elswhere in detail; suffice to say that after his strategy was foiled by Kenshin, Kansuke grabbed a long spear and charged alone into the Uesugi samurai. Having received substantial wounds he disengaged and committed seppuku. The victorious Takeda Shingen had him buried on the field of battle.
It is debated wether Kansuke did everything attributed to him, and even wether he lived at all. It is possible that he was a figure invented purely to populate the pages of the Koyo Gunkan with a true Takeda hero - though current research probably indicates that he did exist.
I have two opposing sources by the same author for Yamamoto Kansuke's uma jirushi (personal banner). This is his uma jirushi depicted in "Samurai Warlords" by Stephen Turnbull.
Uma jirushi (personal banner) or unit nobori (unit standard), and additionally follower sashimono (in reversed colours; black varja on white), from Turnbull's "Samurai Warriors". I tend to trust this less than the other uma jirushi pictured on this site. However, I have used it as a unit nobori in my own Takeda army.
The sashimono (back banner) used by Yamamoto Kansuke's followers. Source: "Samurai heraldry".
Follower sashimono, alternative version: Same as above but white top field.
Kansuke either used a buffalo-horned helmet or a monk's kesa (scarf). The buffalo-horned helmet may well have used this simpler version of the varja.
I have depicted him on foot, wielding a yari and charging headlong forwards, as he did just before his death. He is wearing a monk's kesa. His samurai followers all wear the sashimono with white, black and red varja design. His standard bearer follows him closely, holding his personal uma jirushi aloft. The unit nobori (standard) has a different version of the design used on his uma jirushi.